-LRB- CNN -RRB- -- The Movement for the Emancipation of the Niger Delta has claimed responsibility for an attack on an oil rig and the kidnapping of seven people .

MEND is one of several militant groups in the oil-rich Niger Delta that has been battling the government for years over the distribution of the country 's oil wealth . CNN could not independently confirm the group 's claim .

MEND said in a statement that its fighters carried out the attack on the Transocean rig off the southern coast of Nigeria at Okoro field .

`` All the abducted expatriates are well and in our safe custody , '' the statement said . `` All the fighters who participated in this attack have safely returned to base . Within the next 24 hours , we will release the names of all expatriates currently being held in our different camps . ''

London , England-based Afren PLC said two Indonesians , two Americans , two French and one Canadian were kidnapped . The Indonesians were not included in an earlier tally released by the company .

Two crew members are in stable condition at a shore-based clinic , where they were airlifted after being wounded .

State Department spokesman P.J. Crowley said the United States is in contact with Nigerian officials .

`` We -LSB- are -RSB- , of course , concerned about their safety and hope for their immediate release , '' he said . `` We are working with Nigerian authorities ... to pursue their prompt release , and there 's an investigation already under way . ''

Drilling operations in the Okoro field have been temporarily suspended , both companies said .

In a news release , Afren said a `` security breach '' occurred soon after its High Island VII jackup rig arrived but before drilling had commenced .

`` The situation has not been resolved . Afren is doing everything it can to resolve the situation as quickly as possible , and it is working with relevant agencies in the Nigerian government , '' Afren spokesman James Henderson told CNN .

Transocean operates six offshore rigs in Nigeria -- two deepwater rigs , two ultra-deepwater rigs and two shallow-water rigs , including the one involved in the incident , said Guy Cantwell , a spokesman for the Houston , Texas-based company .

The company has `` taken additional safety precautions '' on its other assets in the west African nation , Cantwell said .

Nigerian officials would not comment about the incident .

CNN 's John Dear and Vivian Kuo contributed to this report .

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NEW : MEND has been battling government over country 's oil wealth

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NEW : `` The seven are well and in safe custody , '' group says

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Drilling operations in the Okoro field have been suspended